1. Field of the Invention
The disclosed invention relates to hydraulic power circuits, in general, and in particular, to quiet and efficient fluid switching.
2. Description of the Related Art
Hydraulic fluid power systems are used or contemplated for use in hybrid vehicle technology as an alternative to electric hybrid systems. Hydraulic hybrid systems have several advantages over electric hybrid systems. For example, electric systems employ electric batteries to store surplus energy. The batteries have limited charge rate capacities and relatively short useful lives. When the batteries are worn out, they must be disposed of, which creates environmental concerns, given the large amounts of heavy metals found in such batteries. For these and other reasons, there is increasing interest in hydraulic hybrid technology.
In fluid power systems, a high-pressure fluid supply line generally includes a shut-off valve between the high-pressure fluid source, such as an accumulator, and the rest of the system. For example, in the case of a system employing an over-center pump/motor, high-pressure fluid is not switched, as in other reversible motors, but is always provided at the same input port of the motor. Thus, high pressure could be provided at that port continually. From a practical standpoint it is more reasonable to shut off high pressure when the motor is at a zero torque output condition, and to maintain the option of closing the line in an emergency. Accordingly, a supply valve is provided in the line for this purpose. It is generally a poppet type check-valve with a pilot control, arranged in the line to permit transfer of fluid up the line to the accumulator while the motor is in pump mode, regardless of the valve position. The valve is designed to withstand extremely high reverse pressure while closed, and to permit a fluid flow down the line exceeding 100 gpm (gallons-per-minute) while open.
FIG. 1 shows a simplified fluid supply circuit 100 for an over-center pump/motor 102 (referred to hereafter as a motor), such as is known in the art. The circuit 100 includes a high-pressure fluid supply 104 and a low-pressure fluid supply 106. A pilot-controlled check valve 108 is positioned between the motor 102 and the high-pressure fluid supply 104, with a fluid supply line 112 extending between the high-pressure fluid supply 104 and the check valve 108, and a fluid supply line 114 between the check valve 108 and the motor 102. A control unit 110 is configured to provide a pilot signal to the check valve 108 via control line 116. The pilot signal may be provided electrically, such as to a solenoid actuator, by fluid pressure, or by other known means.
The motor 102 may be one of a number of types of hydraulic machines, including bent-axis, swash plate, radial piston, etc. For the purpose of the present disclosure, the motor 102 will be considered a bent-axis pump/motor. As is known in the art, the displacement of such bent-axis motors is controlled by changing a stroke angle of the motor. As the angle increases, the output torque of the motor increases. In the case of over-center motors, the angle may be changed in either a positive direction, which applies torque in one direction of rotation, or a negative direction, which applies torque in an opposite direction of rotation. When the motor is at a zero-stroke angle, there is no output torque applied, and no fluid flows through the motor.
In operation, when the motor 102 is to operate in motor mode, the control unit 110 provides the pilot signal to the check valve 108 to open the check valve to fluid flowing from the high-pressure fluid supply 104 to the motor 102. The high-pressure fluid drives the motor 102 in accordance with a selected displacement and direction. When the pilot signal is stopped, the check valve closes to fluid flowing from the high-pressure fluid supply 104 but still permits fluid flowing from the motor 102 to the high-pressure fluid supply 104. When the motor 102 is operated in pump mode, fluid is drawn from the low-pressure fluid supply 106 and pumped to the high-pressure fluid supply 104.